Range finding method

ABSTRACT

The present disclosed subject matter relates to a method for measuring the distance of targets in the surroundings by way of a time-of-flight measurement of pulses reflected at said targets, in particular laser pulses, said method comprising: emitting a sequence of transmission pulses having varying pulse intervals, and receiving at least one receive pulse after each one of two different transmission pulses; for each receive pulse: generating a group of M candidate distances, each based on a different transmission pulse among M transmission pulses preceding the receive pulse, wherein each candidate distance is assigned to the corresponding transmission pulse on which it is based; for each candidate distance: determining a weighting value on the basis of at least the closest of the candidate distances assigned to such a transmission pulse which is adjacent to the transmission pulse to which the candidate distance being considered in this determining process is assigned; for each group: selecting the candidate distance with the highest weighting value as the distance measurement value of the receive pulse for which the group was generated.

The present invention relates to a method for measuring the distance of targets in the surroundings by way of a time-of-flight measurement of pulses reflected at said targets. The pulses can be of any type, for example light pulses, in particular laser pulses, radio pulses, in particular radar pulses, sound pulses, or the like. The invention also relates to a method for laser scanning by continuously directing laser pulses towards different targets in the surroundings.

Modern pulse time-of-flight rangefinders, such as laser rangefinders or scanners, work with high pulse power over large distances and/or high pulse repetition rates in order to quickly create a number of distance measurement points in the surroundings. In both cases the situation can arise that the next pulse is already emitted before the reflection of the last pulse has been received, so that the incoming received pulses can no longer be clearly assigned to their corresponding transmission pulse. This is known as a “multiple time around” (MTA) or “multiple pulses in the air” problem. The maximum size d_(max) of the distance range that can be measured reliably, or what is known as the MTA zone, is given here from the pulse repetition rate (PRR) and the light speed c on the following basis:

$\begin{matrix} {d_{\max} = {\frac{1}{2} \cdot \frac{c}{PRR}}} & (1) \end{matrix}$

Laser scanners of modern design for example offer pulse repetition rates of up to 1200 kHz, which corresponds to an MTA zone size d_(max) of approximately 125 m. If this measurement distance is exceeded, the measurement result generally cannot be correctly interpreted on account of the fact that the transmission and receive pulses cannot be assigned clearly to one another.

FIGS. 1 and 2 show this situation in detail. A pulsed laser measurement beam 2 is guided in a sweeping manner, for example from an airborne laser scanner 1, for example in rows in a fan-shaped manner over a surroundings area U with individual targets in the surroundings (scanning points) U₁, U₂, etc. The distances d₁, d₂, etc. of the individual targets U₁, U₂, etc. in the surroundings are determined from time-of-flight measurements on the individual emitted pulses S₁, S₂, etc., which are retrieved after the reflection at the surroundings as received pulses E₁, E₂, etc.

FIG. 1a and 2a show an exemplary situation when measuring targets in the surroundings U₁, U₂ which are disposed in the first MTA zone Z₁ closest to the laser scanner 1. The receive pulse E₁ belonging to the transmission pulse S₁ is retrieved before the next transmission pulse S₂ is emitted at the time interval τ=1/PRR, and so on and so forth.

FIG. 1b and 2b show an exemplary situation when targets in the surroundings U₃, U₄ are disposed in the second MTA zone Z₂. Here, the receive pulse E₃ belonging to the transmission pulse S₃ is received only once the next transmission pulse S₂ has already been emitted. In order to determine the correct distance d₃ of the target in the surroundings U₃ in the zone Z₂ it is necessary to assign the receive pulse E₃ correctly to the transmission pulse S₃; if the receive pulse E₃ is incorrectly assigned to the directly preceding transmission pulse S₂, an incorrect target distance d₃′ results in the incorrect MTA zone Z₁, instead of the correct target distance d₃ in the correct MTA zone Z₂.

A wide range of different methods are known for mutual MTA-zone-correct assignment of the transmission and receive pulses and thus surmounting of the MTA zone limits for clear distance measurement results; see for example patents AT 510.296, AT 511.310 and AT 515.214 by the same applicant.

The objective of the invention is to further improve the known methods such that they deliver correct distance measurement values also in difficult target situations, such as multiple reflections of a single transmission pulse at targets in the surroundings in different MTA zones or in the case of MTA-zone-breaching jumps in distance in the surroundings to be measured.

This objective is achieved in accordance with the invention by a method for measuring the distance of targets in the surroundings by measuring the time-of-flight of pulses reflected by said targets, in particular laser pulses, said method comprising:

emitting a sequence of transmission pulses having varying pulse intervals, and receiving at least one receive pulse after each one of two different transmission pulses;

for each receive pulse: generating a group of M candidate distances, each based on a different transmission pulse among M transmission pulses preceding the receive pulse, wherein each candidate distance is assigned to the corresponding transmission pulse on which it is based;

for each candidate distance: determining a weighting value on the basis of at least the closest one of the candidate distances assigned to such a transmission pulse which is adjacent to the transmission pulse to which the candidate distance being considered in this determining process is assigned;

for each group: selecting the candidate distance with the highest weighting value as the distance measurement value of the receive pulse for which the group was generated.

The method according to the invention is based on a novel weighting analysis of multiple distance measurement value candidates, referred to here as “candidate distances” for short, which have each been calculated in respect of different preceding transmission pulses of a pulse-position-modulated transmission pulse sequence. The weighting analysis is able to create a highly reliable estimation of the respective MTA-zone-correct distance measurement value for each receive pulse. The method of the invention also delivers excellent MTA zone assignment results even in multi-target situations, in which one transmission pulse results in a plurality of receive pulses, because each receive pulse can be evaluated separately.

A particularly advantageous embodiment of the invention is characterised in that the transmission pulses are emitted with substantially identical amplitude, and for each receive pulse the amplitude thereof is also recorded, and in that the weighting value is formed at least from

a distance weight based on the distance difference between the candidate distance under consideration and said closest candidate distance, and

an amplitude weight based on the amplitude difference between the amplitude of that receive pulse for which the group comprising the candidate distance under consideration was generated and the amplitude of that other receive pulse for which the group comprising said closest candidate distance was generated.

Amplitude values of the receive pulses are thus used for the first time for the MTA-zone assignment or resolution. This is based on the assumption that, for targets in the surroundings with approximately identical reflectivity, receive pulses of targets arranged at a further distance in the surroundings have a lower amplitude than receive pulses of targets arranged closer in the surroundings. By calculating a weighting value for the pairings of candidate distances under consideration for each possible candidate distance based on both the distance difference and the amplitude difference, the distance information hidden in the amplitude of the receive pulses is utilised as additional information for the MTA zone resolution. As a result, the method provides robust, reliable MTA zone assignment results and therefore correct distance measurement values, even in difficult target situations, such as multiple reflections, rapidly changing MTA-zone-breaching jumps in distance in the surroundings, or the like.

The distance and amplitude differences can be weighted in the weighting values for their part in a wide range of different ways so as to produce different response behaviour of the method with respect to multiple reflections and distance jumps. It is preferably provided that the distance difference is incorporated non-linearly into the distance weight, wherein a greater distance different results in an underproportionately smaller distance weight, and the amplitude difference is incorporated non-linearly into the amplitude weight, wherein a greater amplitude difference results in an underproportionately smaller amplitude weight, which results in a particularly robust response behaviour unsusceptible to interference.

In accordance with a first preferred variant of the method, in the aforementioned determining of the weighting value the adjacent transmission pulse is a temporally adjacent transmission pulse. Here, for the weighting value of a candidate distance, preferably a plurality of other candidate distances are taken into consideration on the basis of a plurality of adjacent transmission pulses, more specifically such other candidate distances that are assigned to the transmission pulses directly preceding and directly subsequent in the transmission pulse sequence. The direct temporal proximity of the transmission pulse causal for a receive pulse for the MTA zone assignment is thus examined, which implements the assumption that generally a plurality of successive transmission pulses contact targets in the same MTA zone. Preferably, precisely one temporally preceding and one temporally subsequent transmission pulse and the candidate distances assigned thereto are examined for distance proximity to the respective candidate distance to be weighted and are used for the weighting, i.e. two candidate distance pairings are weighted per candidate distance.

In accordance with an alternative variant of the method according to the invention for scanning a surroundings area in which the transmission pulses are emitted in their temporal sequence to locally different targets in the surroundings, in the aforementioned determining of the weighting value the adjacent transmission pulse is a transmission pulse locally adjacent in respect of the targets in the surroundings. This embodiment takes into account the fact that, when scanning a surroundings area, transmission pulses temporally successive in the transmission pulse sequence do not necessarily occur locally adjacently in the surroundings, for example if laser pulses are guided in a manner scanning the surroundings in rows with a polygon deflection mirror. Laser pulses occurring locally adjacently in the surroundings can, rather, also originate from transmission pulses which are not directly successive in the transmission pulse sequence, but instead are successive for example at a distance from a scanning row or scanning period. The term “adjacent” transmission pulse is understood accordingly in the present description as a comprehensive term for the two variants of a temporally adjacent transmission pulse or a locally adjacent transmission pulse.

In the latter variant a plurality of transmission pulses locally adjacent to the transmission pulse of the candidate distance to be weighted are preferably used for the determining of the weighting value. To this end, the weighting value is formed from partial weights, wherein each partial weight is based on the closest one of the candidate distances assigned to the respective locally adjacent transmission pulse. The partial weights are then summed for example to give the weighting value, and the candidate distance weighted in this way thus takes into consideration the local impingement surroundings so to speak of its original transmission pulse for the MTA zone resolution.

In this embodiment as well, a significant increase in the accuracy of the MTA zone assignment and thus robustness and precision of the distance measurement method can be attained if on the precondition that the transmission pulses are emitted with substantially the same amplitude and for each receive pulse the amplitude thereof is also recorded the aforesaid partial weights are composed again from a distance weight and an amplitude weight. In this way the amplitude of the receive pulses is utilised for the first time for the MTA zone assignment of the receive pulses under consideration of the local impingement surroundings of each transmission pulse. This embodiment also is based on the knowledge that adjacent transmission pulses impinging on a target usually experience the same reflectivity, and therefore valuable additional information for the MTA zone resolution can be obtained from the amplitude of the receive pulses.

The distance and amplitude differences in the partial weights of the weighting value can be provided again with corresponding non-linear weighting functions in order to increase the robustness of the method.

In accordance with a further preferred feature of the invention, in each of the mentioned embodiments when determining the weighting value only those closest candidate distances that lie within a predefined distance range around the considered candidate distance can optionally also be taken into consideration, which saves computing time when calculating the weighting values.

The number M of candidate distances of a group which is generated for a receive pulse defines the number of possible MTA zones which can be assigned (“resolved”) with the method. When generating a group, the M candidate distances are preferably based on M transmission pulses directly preceding the receive pulse, whereby M MTA zones directly adjacent to the emission location of the transmission pulses can be measured and resolved.

For the correct assignment (resolution) of M MTA zones it is sufficient if, during emission, the pulse distances are varied in accordance with a repeating code, the code length of which is greater than or equal to M. For example, a variation of the pulse intervals which is repeated after every 7 pulse intervals, i.e. a code of code length 7, is thus sufficient for the resolution of 7 MTA zones.

The invention will be explained in greater detail hereinafter on the basis of exemplary embodiments illustrated in the accompanying drawings. In the drawings:

FIG. 1 schematically shows various reflection situations of a pulsed laser scanning beam at targets in the surroundings which lie in various MTA zones, according to the prior art;

FIG. 2 shows exemplary time graphs of transmission and receive pulses for the reflection situations of FIG. 1, according to the prior art;

FIG. 3 shows a multi-target situation in a schematic perspective view;

FIG. 4 shows a schematic block diagram of a laser scanner for carrying out the method of the invention;

FIG. 5 shows exemplary time graphs of transmission and receive pulses within the scope of the method of the invention;

FIG. 6 shows a flow diagram of the method of the invention;

FIG. 7 shows a combined time and assignment graph for transmission and receive pulses within the scope of the method of the invention;

FIG. 8 shows locally adjacent transmission pulses impinging on a target surroundings during laser scanning according to the method of the invention;

FIGS. 9a and 9b show exemplary weighting functions for distance and amplitude differences within the scope of the method of the invention; and

FIGS. 10a and 10b show exemplary 3D point clouds of distance measurement points of a target surroundings, created once with a method according to the prior art (FIG. 10a ) and once with a method according to the invention (FIG. 10b ).

FIGS. 1 and 2 show the pulse assignment problem of MTA-zone-exceeding distance measurement or scanning regions already explained in the introduction. This problem is intensified in what are known as multi-target situations according to FIG. 3, where a single transmission pulse is reflected by a plurality of targets U₁, U₃ in the surroundings arranged one after the other, possibly also in different MTA zones Z₁, Z₂. A transmission pulse S₁ of the laser measurement beam 2 considered to be representative experiences for example a first reflection at a close target U₁ in the first MTA zone Z₁, for example a power line, foliage or the like, which it merely brushes against, or a semi-transparent intermediate target, such as a cloud, a glass pane, etc.; and a second reflection at a distant target U₃ in the same or a different MTA zone, here the second MTA zone Z₂. The laser scanner 1 in such a situation receives, for the transmission pulse S₁, two receive pulses E₁, E₃. In the case of foliage, woods, etc. three, four or more receive pulses E_(i) can also be received per transmission pulse S_(p) (i, pϵN). If the laser rangefinder or scanner 1 is able to record and process more than one receive pulse E_(i) per transmission pulse S_(p), in particular also between two transmission pulses S_(p), it is referred to as “multi-target-enabled”. It is evident that the correct MTA zone assignment of a receive pulse E_(i) in a multi-target-enabled rangefinder or scanner is much more difficult than as shown in FIGS. 1 and 2.

In order to solve the stated MTA zone assignment problem, the method described now with reference to FIGS. 4 to 10 is used. The method will be described on the basis of a multi-target-enabled laser scanner, although this is not absolutely necessary. The method can thus also be used for automatic MTA zone assignment (“MTA resolution”) in laser scanners that are not multi-target-enabled, that is to say also in simple laser rangefinders for which the transmission pulses are not scanned over the surroundings, but are directed continuously towards the same target in the surroundings. Lastly, the described method is suitable not only for distance measurements by measuring the time-of-flight of laser pulses, but also of any pulses, whether these be radio pulses, in particular radar pulses, sound or sonar pulses, electrical pulses over electrical lines, for example for line length measurement, etc.

As shown in FIG. 4 and the upper graph of FIG. 5, a multi-target-enabled laser scanner 3 transmits transmission pulses S₁, S₂, etc., generally S_(p), with a substantially constant amplitude a_(s) at successive transmission times τ_(S,1), τ_(S,2), etc., generally τ_(S,p), by means of a laser transmitter 4. The pulse intervals τ₁=τ_(S,2)−τ_(S,1), τ₂=τ_(S,3)−τ_(S,2), generally τ_(p)=τ_(S,p+1)−τ_(S,p), of the transmission pulses S_(p), vary from pulse to pulse, more specifically either randomly or preferably in accordance with a repeating pattern or “code” C with a pattern or code length L. In the shown example the code length is L=5, i.e. after five different pulse intervals τ₁, τ₂, τ₃, τ₄, TS the sixth pulse interval τ₆ is the same again as the first pulse interval τ₁, and so on and so forth. A pulse interval variation of this kind is also referred to as pulse position modulation (PPM) inasmuch as the individual pulse positions (transmission times) t_(S,p) are “pulse-position-modulated” in their time position relative to the cycle of a constant pulse repetition rate (PRR=1/τ with the code C).

The transmission pulses S_(p) are guided over the surroundings U from the laser transmitter 4 via a semi-permeable mirror 5 and a deflection device 6, for example a rotating polygon mirror wheel, as laser measurement beam 2 oscillating to and fro, and are reflected there by a target U_(p) in the surroundings and are guided back again via the deflection device 6 to the semi-permeable mirror 5, pass through this and impinge on a laser receiver 7. The laser receiver 7 detects each incoming receive pulse E_(i) and measures the receive time t_(E,i) and amplitude a_(i) thereof. In the lower graph of FIG. 5 a sequence of such receive pulses E_(i) is shown by way of example with their receive times t_(E,i) and amplitudes a_(i).

Both the transmission times τ_(S,p) of the transmission pulses S_(p) and the 2-tuple (t_(E,i), a_(i)) of receive times t_(E,i) and amplitudes a_(i) of the receive pulses E₁ are supplied to a processor 8 and are stored thereby for example in a memory 8′. The processor 8 with use of the subsequently described method by MTA-zone-correct assignment of each receive pulse E_(i) or 2-tuple (t_(E,i), a_(i)) to the transmission pulse S_(p) or transmission time t_(S,p) causal therefor calculates the time of flight

ΔT _(i) =t _(S,p) −t _(E,i)  (2)

and on this basis, as is known, the distance

d _(i) =ΔT _(i) ·c/2.  (3)

Since the times of flight ΔT_(i) and the distance measurement values d_(i) of the targets in the surroundings are proportional to one another, times of flight and distances in the present description are also used synonymously and exchangeably.

In order to assign each receive pulse E_(i) the “correct” causal transmission pulse S_(p) for the distance measurement, or conversely to determine for a transmission pulse S_(p) the “correct” receive pulse or—in multi-target situations—receive pulses E_(i) from the sequence of receive pulses {E_(i)} and on this basis ultimately the correct distance measurement value d_(i) for each target in the surroundings U_(p), the processor 8 performs the method described with reference to FIGS. 6 to 9.

The first step 9 of the method of FIG. 6 relates to the emission, just described, of the sequence {S_(p)} of transmission pulses S_(p) with varying pulse intervals τ_(p) (FIG. 5) and the accompanying receiving and recording of the sequence {E_(i)} of receive pulses E_(i) and measurement of the receive times t_(E,i) and amplitudes a_(i) thereof. If the method is performed in a laser scanner 3 with a beam deflection device 6 which scans the laser beam 2 over the surroundings U, for example in adjacent scanning rows 10, as shown later in FIG. 8, such that the temporally successive transmission pulses S_(p) also give a locally distributed pattern of targets U_(p) in the surroundings U impinged by the transmission pulses S_(p), this scanning is performed likewise in step 9.

In a next step 11 a group G_(i) of M distance measurement value candidates, referred to as “candidate distances” for short, d_(i,m), with m=1 . . . M, is now generated for each receive pulse E_(i). The number M defines the number of MTA zones Z_(r) which can be resolved, i.e. in which the distance of targets in the surroundings U_(p) can be measured with correct MTA zone assignment. For this function it is also necessary that the code length L of the pulse distance variation (pulse position modulation) of the transmission pulses S_(p) is greater than or equal to M.

Each candidate distance d_(i,m) of a group G_(i) of a receive pulse E_(i) is based here on another of M transmission pulses S_(p) preceding the receive pulse E_(i), i.e. was calculated from the time of flight between the receive time t_(E,I) of this receive pulse E_(i) and the transmission time τ_(S,p-m) of the respective transmission pulse S_(p-m) to which reference was made for this candidate distance d_(i,m). This is explained in detail on the basis of the graph of FIG. 7.

As an example, reference is made to the receive pulse E₆ in FIG. 7, which was received directly after the seventh transmission pulse S₇. The horizontal, solid lines 12 of the graph of FIG. 7 each represent time axes starting at the transmission time t_(S,p) of a transmission pulse S_(p)—and thus referred to synonymously as: distance axes—on which—similarly to FIG. 5—the receive pulses E_(i) arriving after one transmission pulse S_(p) up until the next transmission pulse S_(p+), have been plotted. Once the transmission pulse S₄ of the first receive pulse E₁ had been received, the transmission pulse S₅ was then emitted, then the receive pulse E₂ was received, and then the transmission pulse S₆ was emitted, whereupon three receive pulses E₃, E₄, E₅ were received before the next transmission pulse S₇ was emitted, whereupon the receive pulse E₆ which is exemplary here was received, and so on and so forth. The vertical distance between two time or distance axes 12 corresponds in the graph of FIG. 7 to the respective pulse distance τ_(p).

The group G₆ for the receive pulse E₆ is composed in the example of FIG. 7 of M=4 candidate distances d_(6,1), d_(6,2), d_(6,3) and d_(6,4). In the graph of FIG. 7 the groups G_(i) are each symbolised by a dashed line 13.

The candidate distances d_(6,1) to d_(6,4) are each calculated on the basis of the time difference between the receive time t_(E,6) of the receive pulse E₆ and the respective transmission time τ_(S,7), τ_(S,6), τ_(S,5) and τ_(S,4) of the M=4 previous transmission pulses S₇, S₆, S₅ and S₄ to give:

d _(6,1)=(t _(E,6) −t _(S,7))·c/2

d _(6,2)=(t _(E,6) −t _(S,6))·c/2

d _(6,3)=(t _(E,6) −t _(S,5))·c/2

d _(6,4)=(t _(E,6) −t _(S,4))·c/2  (4)

As can be seen from FIG. 7, with the generation of the groups G_(i) in step 11, each candidate distance d_(i,m), is at the same time assigned to the transmission pulse S_(p) on which it is based, i.e. in the present example:

d _(6,1) →S ₇

d _(6,2) →S ₆

d _(6,3) →S ₅

d _(6,4) →S ₄  (5)

This is symbolised in FIG. 7 in that the candidate distances d_(i,m), of the groups G_(i) are plotted on the time or distance axis 12 of the transmission pulse S_(p) to which they were assigned, whereby the slanted course of the group lines 13 results.

Each candidate distance d_(i,m), or each 2-tuple (d_(i, m), a_(i)) is thus simultaneously assigned a transmission pulse index, generally p, and thus gives the 2-tuple (d_(i,m), p) or 3-tuple (d_(i,m), a_(i), p) respectively. The amount {(d_(i,m), p)} or {(d_(i,m), a_(i), p)} of 2-tuples or 3-tuples generated in step 11 is stored again in the memory 8′, for example.

In the next step 14 (FIG. 6), a weighting value W_(i,m) is now determined as follows for each candidate distance d_(i,m), of this amount. The weighting value W_(i,m) is determined on the basis of at least one “pairing” of the respective candidate distance d_(i,m), under consideration to be weighted and at least one “neighbour” candidate distance d_(j,n). The neighbour candidate distances d_(j,n) eligible for the pairings, in the graph of FIG. 7, lie in a “catch region” 15 around the considered candidate distance d_(i,m), which is defined by the following criteria:

(1) The neighbour candidate distance d_(j,n) in the catch region 15 is assigned a transmission pulse S_(p±1) (here: the transmission pulses S₅ and S₇) which is adjacent to the transmission pulse S_(p) (here: S₆) to which the considered candidate distance d_(i,m), to be weighted (here: d_(6,2)) is assigned. A transmission pulse “adjacent” to a transmission pulse S_(p) is understood here to be both a temporally adjacent transmission pulse S_(p±1), S_(p±2), etc., for example in this case the temporally preceding transmission pulse S₅ or the temporally subsequent transmission pulse S₇, or a locally adjacent transmission pulse S_(p±x) (xϵN), as shown in FIG. 8.

FIG. 8 shows a local catch region 15 for the transmission pulse S₆ considered here by way of example, which is assigned the exemplary candidate distance d_(6,2), which is exemplary here. In the catch region 15, the transmission pulses S⁻⁹⁹⁵, S⁻⁹⁹⁴, S⁻⁹⁹³, S₅, S₇, S₁₀₀₅, S₁₀₀₆ and S₁₀₀₇ are locally adjacent to the transmission pulse S₆ if the catch region 15 has a size of 3×3 transmission pulses S_(p). Catch regions 15 of other sizes, for example 4×3, 4×4, 5×3, 5×4, 5×5 etc., are also possible. It is clear that transmission pulses S_(p) from different scanning rows 10 can have a large temporal distance from one another, here for example a distance of 1000 intermediate transmission pulses, and yet can still be locally adjacent to one another in the catch region.

(2) The second criterion for adjacent candidate distances d_(j,n), which thus at the same time defines the catch region 15, lies in that these candidate distances d_(j,n) must be the closest of the candidate distances assigned to a (temporally or locally) adjacent transmission pulse of this kind. In the example of FIG. 7 the candidate distances d_(2,1), d_(3,2), d_(4,2), d_(5,2), d_(6,3), d_(7,3) and d_(8,4) are assigned to the exemplary neighbour transmission pulse S₅ (temporal neighbour of the transmission pulse S₆ to which the candidate distance d_(6,2) of the receive pulse E₆ is assigned), and, of these, the one with the distance value closest to the considered candidate distance d_(6,2) is the candidate distance d_(3,2).

Optionally, it can also be provided in the criterion (2) that candidate distances d_(j,n) which indeed satisfy criterion (2) but lie outside a predefined distance range (synonym: time range) around the considered candidate distance d_(j,n) (here: d_(6,2)) are not taken into consideration, A distance range of this kind can be seen in the graph of FIG. 7 as a horizontal width b of the catch region 15 on the time or distance axes 12; all candidate distances d_(j,n) outside the width b of the catch region 15 remain out of consideration throughout the rest of the process.

All of the candidate distances d_(j,n) which satisfy the two above criteria (1) and (2), i.e. qualify for the catch region 15 or thus define it, are taken into consideration in step 14 for the determining of the weighting value W_(i,m) of the considered candidate distance d_(i,m). If just one qualifying candidate distance d_(j,n) lies in the catch region 15, for example if the catch region 15 is defined to be so small that only one neighbour transmission pulse is considered and the width b is small, the weighting value W_(i,m) is then composed exclusively from a single partial weight for the pairing d_(i,m)⇄d_(j,n). If a plurality of qualifying candidate distances d_(j,n) lie in the catch region 15, a partial weight PW_(i,m,k) (k=1 . . . K) is calculated for each of K possible pairings 16 between the considered candidate distance d_(i,m) and the respective candidate distance d_(j,n),k paired therewith, and the weighting value W_(i,m) of the candidate distance d_(i,m) is given as

$\begin{matrix} {W_{i,m} = {\sum\limits_{k}{PW}_{i,m,k}}} & (6) \end{matrix}$

with

PW _(i,m,k) =f ₁(d _(i,m) ,d _(j,n,k))  (7)

or

PW _(i,m,k) =f ₂((d _(i,m) ,a _(i)),(d _(j,n,k) ,a _(j)))  (8)

With k=1, i.e. only one pairing 16, the partial weight PW_(i,m,k) corresponds directly to the weighting value W_(i,m). With k>1, the K partial weights PW_(i,m,k) can also for their part be incorporated into the weighting value W_(i,m) in differently weighted form, for example in order to weight diagonal pairings 16 in a square local catch region 15, such as that of FIG. 8, lower than vertical or horizontal pairings 16.

In the function f₁ of equation (7), each partial weight PW_(i,m,k) considers the distance difference between the considered candidate distance d_(i,m) and the paired candidate distance d_(j,n,k), i.e.

PW _(i,n,k) =EG _(i,m,k) =f ₁(d _(i,m) ,d _(j,n,k))=f _(EG)(d _(j,n,k) −d _(i,m))  (9)

FIG. 9a shows an example of a distance weight function f_(EG) of this kind, which converts the distance difference d_(j,n,k)−d_(i,m) of the candidate pairing 16, plotted on the x-axis of the graph of FIG. 9a and standardised to the catch region width b, into a distance weight EG_(i,m,k), plotted on the y-axis of the graph of FIG. 9a . FIG. 9a shows four different variants v_(i), v₂, v₃ and v₄ of the distance weight function f_(EG) with linear (v₁) or increasingly severe, non-linear drop with greater difference values (v₁, v₂, v₃).

The partial weight PW_(i,m,k)—and thus ultimately the weighting value W_(i,m)—is optionally and preferably formed additionally on the basis of the amplitude values a_(i) and a_(j) of the candidate distances d_(i,m) and d_(j,n) involved in the respective pairing 16, as can be seen by the function f₂ in equation (8). To this end the amplitude difference a_(j,n,k)−a_(i,m) of the candidate distances d_(i,m) and d_(j,n) involved in the pairing 16 is firstly calculated with an amplitude weight function f_(AG) to give an amplitude weight AG_(i,m,k) on the following basis:

AG _(i,m,k) =f _(AG)(a _(j,n,k) −a _(i,m))  (10)

FIG. 9b shows four exemplary variants v₁, v₂, v₃ and v₄ of an amplitude weight function f_(AG) of this kind, wherein again the amplitude difference a_(j,n,k)−a_(i,m) is plotted on the x-axis (in dB) and the amplitude weight AG_(i,m,k) is plotted on the x-axis, more specifically in four different variants v₁, v₂, v₃, v₄ with (on a logarithmic scale) linear (v₁) or increasingly more severe, non-linear drop with greater difference values (v₂, v₃, v₄).

The partial weight PW_(i,m,k) of the k^(th) pairing 16 is then calculated from the sum of any function or preferably a product of the distance weight EG_(i,m,k) and the amplitude weight AG_(i,m,k) on the following basis:

PW _(i,m,k) =EG _(i,k) ·AG _(i,m,k)  (11)

The partial weights PW_(i,m,k) are then summed, as explained above, to give the weighting value W_(i,m):

$\begin{matrix} {W_{i,m} = {\sum\limits_{k}{PW}_{i,m,k}}} & (6) \end{matrix}$

Once in step 14 (FIG. 6) weighting values W_(i,m) have been calculated in this way for all candidate distances d_(i,m), the candidate distances d_(i,m) in each group G_(i) which have the maximum weighting value W_(i,m) in the group G_(i) in question are selected in a subsequent step 17. The candidate distance d_(i,m) selected in a group G_(i) now represents the distance measurement value d_(i) of the receive pulse E_(i) for which the group G_(i) was generated:

d _(i) ={d _(i,m)|max(W _(i,m))}  (12)

A distance measurement value d_(i) which is optimally MTA-zone-correct is thus now determined for each receive pulse E_(i).

FIG. 10 shows the performance of the presented method on the basis of two examples of a 3D point cloud of distance measurement points of a surroundings area, more specifically once with a conventional laser scanner 1 (FIG. 10a ) and once with a laser scanner 3 operating in accordance with the presented method ((FIG. 10b ).

It is clear from FIG. 10a that a ladder 17 set up in the MTA zone Z₁ leads to massive assignment errors of a building façade 18 located therebehind in the MTA zone Z₂, see the artefacts of a façade element 19 incorrectly assigned to the MTA zone Z₁ and thus appearing to be in the vicinity of the ladder 17.

In the example of FIG. 10b the described method was performed with weighting values from eight partial weights each formed from distance and amplitude weights for a local 3×3 catch region 15. The method led to a correct assignment of the entire building façade 18 to the second MTA zone Z₂ lying far behind the ladder 17. The shadow of the ladder 17 on the building façade 18 is thus clearly discernible, without parts of the building façade 18 having been assigned incorrectly to the MTA zone Z₁.

The invention is not limited to the presented embodiments, but instead comprises all variants, modifications and combinations that fall within the scope of the accompanying claims. 

What is claimed is:
 1. A method for measuring a distance of a target in surroundings by measuring the time-of-flight of pulses reflected by said target, said method comprising: emitting a sequence of transmission pulses having varying pulse intervals, and receiving at least one receive pulse after each one of two different transmission pulses; for each receive pulse: generating a group of M candidate distances, each based on a different transmission pulse among M transmission pulses preceding the receive pulse, wherein each candidate distance is assigned to the corresponding transmission pulse on which it is based; for each candidate distance: determining a weighting value on the basis of at least a closest one of the candidate distances assigned to such a transmission pulse which is adjacent to the transmission pulse to which the candidate distance being considered in this determining process is assigned; for each group: selecting the candidate distance with a highest weighting value as distance measurement value of the receive pulse for which the group was generated.
 2. The method according to claim 1, wherein the transmission pulses are emitted with substantially identical amplitude, and for each receive pulse an amplitude thereof is also recorded, and in that the weighting value is formed at least from a distance weight based on a distance difference between the candidate distance under consideration and said closest candidate distance, and an amplitude weight based on an amplitude difference between the amplitude of that receive pulse for which the group comprising the candidate distance under consideration was generated and the amplitude of that other receive pulse for which the group comprising said closest candidate distance was generated.
 3. The method according to claim 2, wherein the distance difference is incorporated non-linearly into the distance weight, wherein a greater distance different results in an underproportionately smaller distance weight, and the amplitude difference is incorporated non-linearly into the amplitude weight, wherein a greater amplitude difference results in an underproportionately smaller amplitude weight.
 4. The method according to claim 1, wherein when determining the weighting value said adjacent transmission pulse is a temporally adjacent transmission pulse.
 5. The method according to claim 4, wherein the weighting value is determined based at least on a closest one of the candidate distances assigned to that transmission pulse which temporally precedes the transmission pulse to which the candidate distance considered for this determination is assigned, and a closest one of the candidate distances assigned to that transmission pulse which temporally follows the transmission pulse to which the candidate distance considered for this determination is assigned.
 6. The method according to claim 1 for scanning a surroundings area, wherein the transmission pulses are emitted in their temporal sequence to locally different targets in the surroundings, wherein in the determination of the weighting value said adjacent transmission pulse is a transmission pulse locally adjacent in respect of the targets in the surroundings.
 7. The method according to claim 6, wherein a plurality of locally adjacent transmission pulses are used for the determination of the weighting value, in that the weighting value is formed from partial weights, and in that each partial weight is based on a closest one of the candidate distances assigned to the respective locally adjacent transmission pulse.
 8. The method according to claim 7, wherein the transmission pulses are emitted with substantially identical amplitude and for each receive pulse the amplitude thereof is also recorded, and in that each partial weight is formed at least from a distance weight based on a distance difference between the candidate distance under consideration and the aforementioned respective closest candidate distance, and an amplitude weight based on an amplitude difference between the amplitude of that receive pulse for which the group comprising the candidate distance under consideration was generated, and the amplitude of that other receive pulse for which the group comprising the aforementioned respective closest candidate distance was generated.
 9. The method according to claim 8, wherein the distance difference is incorporated non-linearly into the distance weight, wherein a greater distance difference results in an underproportionately smaller distance weight, and in that the amplitude difference is incorporated non-linearly into the amplitude weight, wherein a greater amplitude difference results in an underproportionately smaller amplitude weight.
 10. The method according to claim 1, wherein when determining the weighting value only the closest candidate distances which lie within a predefined distance range around the considered candidate distance are taken into consideration.
 11. The method according to claim 1, wherein when generating the group said M candidate distances are based on M transmission pulses directly preceding the receive pulse.
 12. The method according to claim 1, wherein, during emission, the pulse distances are varied in accordance with a repeating code, the code length of which is greater than or equal to M.
 13. The method of claim 1, wherein the pulses are laser pulses.
 14. The method according to claim 2, wherein when determining the weighting value said adjacent transmission pulse is a temporally adjacent transmission pulse.
 15. The method according to claim 14, wherein the weighting value is determined based at least on a closest one of the candidate distances assigned to that transmission pulse which temporally precedes the transmission pulse to which the candidate distance considered for this determination is assigned, and a closest one of the candidate distances assigned to that transmission pulse which temporally follows the transmission pulse to which the candidate distance considered for this determination is assigned. 